There are already known measuring arrangements which generally use a grip current tester or a torus which may or not be opened and which fully closes about a cable in which the current to be measured is flowing.
Now it is known that the presence of discontinuities in the magnetic circuit, as produced by an air gap, a winding, an opening of the grip current tester for tightly enclosing the cable, the insertion of the sensor or the like creates zones on torus which exhibit a greater sensitivity to magnetic fields of external origin, i.e., a parasitic field thereby altering or making false the measurement of the current.
The usual measures for decreasing these external influences consist either in reducing these discontinuities to the utmost or in modifying the geometry of the parts present in this sensitive zone or in adding a shield or screen in the vicinity of this zone.
These different measures for solving the problem caused by the discontinuities are the major inconveniences likely to change the design of the measuring equipment and of adding difficulties during its manufacture.